The invention, in general, relates to the field of the manufacturing integrated semiconductor circuits such as VLSI and ULSI circuits using photolithographic methods. In particular, the invention relates to increasing the resolution of conventional photolithography by using alternating phase masks.
When integrated semiconductor circuits are manufactured, the mask structures that are assigned to the circuit elements are conventionally optically imaged on light-sensitive layers on the wafer. Because of reflective effects, the resolution of such an imaging system is limited and mask structures with dimensions below the reciprocal value of this resolution, also referred to as critical structures, are imaged in a smeared fashion or are out of focus. This leads to undesired strong correlations between the circuit elements and this causes the functionality of the circuits to be impaired.
These difficulties can be overcome by utilizing the destructive interference effect of two directly adjacent and coherent light beams with phases which are shifted through 180xc2x0, and by converting the conventional masks in question into alternating phase masks in which each critical structure is provided with two phase shifters for generating the necessary phase shift.
Various types of phase masks are described, for example, in the book xe2x80x9cTechnologie hochintegrierter Schaltungenxe2x80x9d [Technology of highly integrated circuits] by D. Widmann, H. Mader and H. Friedrich, 2nd edition, Springer-Verlag [publishing house], pages 135 et seq. A detailed summary of phase mask technology is given in the publications xe2x80x9cImproving Resolution in Photolithography with a Phase-Shifting Maskxe2x80x9d by M. D. Levenson et al. in IEEE Trans. Electron. Devices 29 (1982), pages 1828 et seq. and xe2x80x9cWavefront Engineering for Photolithographyxe2x80x9d by M. D. Levenson in Physics Today, July 1993, pages 28 et seq.
The use of what are referred to as strong phase masks, which include both the alternating phase masks already mentioned and chromium-free phase masks, requires the transparent phase-shifting structures in each affected plane to be assigned to one of two phases which have a phase difference xcex94xcfx86=180xc2x0. Here, the following two cases must be distinguished: in what is referred to as a dark-field phase mask, transparent structures correspond to the circuit elements (for example conductor tracks) and phases can be assigned to them, while non-transparent mask fields are formed by regions covered with chromium. In contrast, in what is referred to as a bright-field phase mask, the non-transparent regions of the phase mask, which are covered with chromium and which constitute the circuit elements, and the regions lying between them are transparent. In the latter case, suitable regions in the vicinity of the non-transparent chromium regions must be defined as phase-shifting elements. The phase-shifting elements are produced in accordance with specific design rules, known per se in the prior art. The method of production is described, for example, in U.S. Pat. No. 5,537,648, which is incorporated herein by reference.
In view of the complexity of modern circuits and the requirement for two phase-shifting elements which are displaced by 180xc2x0 on each critical structure, it is, however, conceivable that there will be phase conflicts. A phase conflict occurs precisely if the same phase is incorrectly assigned to the phase shifters on both sides of a critical structure or if, because of the interaction of the phase-shifting elements, the destructive interference effect occurs at an undesired point on the aforementioned light-sensitive layer. The phase assignment for the various phase-shifting elements thus constitutes a mathematical combinatorial problem which cannot be generally solved. Because the phase assignment can in principle lead to different results and different phase assignments can take place for one and the same cell of a hierarchical layout, the phase assignment must be performed last on the finished circuit layout in an automated program. For this reason, there is a need for an automated checking routine that examines a circuit layout to determine whether phase assignment is at all possible. This checking should be complete and should restrict the problem as satisfactorily as possible, i.e. should determine its actual place of origin. The latter is not self-evident; it is due to the fact that if the combinatorial function xe2x80x9cdoes not goxe2x80x9d, this is possible in various ways and the point at which it is discovered that this is the case can lie far from the actual place of origin.
After phase conflicts have been determined in an automated routine, they can be resolved in basically two different ways. First, the circuit design can be slightly changed at the points of localized phase conflicts, for example, by shifting conductor track structures so that the phase conflicts are eliminated. On the basis of this changed circuit design, a successful phase assignment can then be carried out in order to generate a phase mask. Second, the circuit design can remain unchanged and instead the phase conflicts can be resolved by assigning two different phases to individual phase-shifted elements. However, the result of this is that a dark line occurs in the exposure on the boundary between the two different phase regions, which would lead to an interruption. For this reason, in this case an additional exposure step is carried out with what is referred to as a trim mask which is used for specially exposing the dark lines which occur.
In the prior art, two different methods are known for checking a layout for phase conflicts. The publication xe2x80x9cHeuristic Method for Phase-Conflict Minimization in Automatic Phase-Shift Mask Designxe2x80x9d by A. Moniwa et al. in Jpn. J. Appl. Phys., Vol. 34 (1995), pp. 6584-6589, discloses an access method based on graph theory in which a set of phase-shifting elements are postulated and a planar, non-directed graph is formed from this set taking into account the technological requirements. In this method based on graph theory, graph nodes (vertices) constitute phase-shifting elements. A graph edge between two nodes means that the region between the associated phase shifters is lithographically critical. In this method phase conflicts are manifested as those cycles with an uneven number of vertices. Because of the significance of the graph edges, a break in a cycle, i.e. resolution of a phase conflict is equivalent to expanding the corresponding critical region. An efficient conflict resolving strategy in accordance with the aforesaid method should be to break the edges occurring most frequently in the uneven cycles.
U.S. Pat. No. 5,923,566 describes a computer-implemented routine which is used to verify whether an existing circuit design can be imaged onto a phase mask or whether localized phase conflicts are present. The phase conflicts are registered from the interaction between critical circuit regions and the coherent free circuit regions which are to be determined taking into account the technological requirements. Free circuit regions with an uneven number of interactions represent the phase conflicts.
Both methods described above do not, however, operate in an optimum way in registering phase conflicts. First, as is explained below with reference to examples, these two methods prove inefficient because, for example, certain phase conflicts are indicated in duplicate. Second, they prove inadequate because they cannot be used to register other phase conflicts.
Therefore, the phase conflicts cannot be correctly registered by means of the identification methods which are known from the prior art. Consequently, even a conflict elimination method which uses the results of the identification method to eliminate the identified phase conflicts cannot give optimum results.
It is accordingly an object of the invention to provide a method for detecting possible phase conflicts on alternating phase masks and for automatically eliminating these detected phase conflicts which overcomes the above-mentioned disadvantages of the prior art methods of this general type. In particular, it is an object of the invention to provide such a method having a first step in which, a set of phase conflicts that is present is completely determined in a minimal fashion, and having a second step in which the set of phase conflicts can be eliminated in an automated fashion by exclusively using the technological requirements which are made of the circuit structure. After the elimination of the phase conflicts, a layout for a phase mask can then be generated.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for detecting possible phase conflicts on alternating phase masks and for automatically eliminating these detected phase conflicts that is applied to a dark-field phase mask, that is to say circuit elements such as electrical conductor tracks are to be imaged onto transparent regions of the phase mask.
In this method:
In a method step a.) critical regions are determined in which in each case two adjacent transparent regions which are provided for the phase mask are less than a specific predefined minimum distance from one another.
In the next method step b.) overlapping regions between straight sections of the critical regions obtained and end regions of straight sections of the critical regions which end in the middle of transparent regions are determined, and critical regions which are degenerated are produced. The latter are obtained by removing overlapping regions from the critical regions.
In a further method step c.) coherent regions (lands) which lie outside the transparent regions and the critical regions are then determined, and large outer borders of the lands and of the overlapping regions and end regions obtained in the preceding method step are determined. Then, in a method step the number of contact lines between each of the specific outer borders and the degenerated critical regions is determined, and a phase conflict is detected if the number is uneven.
Finally, in a method step e) the phase conflicts are resolved in that coherent layout regions and the region borders of the regions are determined, coherent layout regions are defined by the transparent regions and the critical regions which lie between them, minus the end regions and the overlapping regions which represent phase conflicts (A), at least one connecting path is generated between a large outer border which represents a phase conflict and either one directly adjacent region border which lies further out or an uneven number of large outer borders which are not yet connected and which also represent phase conflicts (B), the set of connecting paths generated is then reduced to that in which each phase conflict is contained precisely once (C), those regions (coverage regions) of the connecting paths which lie over the transparent regions are then marked (D) and finally, for the phase mask, the coverage regions are formed as region boundaries between two different regions of the phase mask which is to be manufactured and whose phase shifts have a phase difference of xcex94xcfx86=180xc2x0 (E).
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for detecting possible phase conflicts on alternating phase masks and for automatically eliminating these detected phase conflicts that is applied to a bright-field phase mask, that is to say circuit elements such as electrical conductor tracks are to be imaged onto non-transparent regions of the phase mask. In this method:
In a method step a.) phase-shifting regions are determined on each side of n on-transparent critical regions which are provided for the phase mask. Critical regions are defined by the fact that they are less than a predefined structural width.
Then , in a method step b.) overlapping regions between straight sections of the critical regions and end regions of straight sections of the critical regions which end in the middle of phase-shifting regions or interaction regions between phase-shifting regions are determined, and degenerated critical regions are produced. The latter are obtained by removing overlapping regions from the critical regions .
In a method step c.), coherent regions which lie outside the phase-shifting and critical regions are determined, and large external borders of the se lands and of the overlapping regions and end regions obtained in the preceding method step are determined.
Finally, in a method step d.), the number of contact lines between each of the specific outer borders and the degenerated critical regions is determined and a phase conflict is detected if the number is uneven.
In a method step e.) the phase conflicts are then resolved in that coherent layout regions and the region borders (40) of the regions are determined, coherent layout regions are defined by the phase-shifting regions, the critical regions and the interaction regions, minus the end regions and the overlapping regions which represent phase conflicts (A), at least one connecting path being generated between a large outer border which represents a phase conflict and either one directly adjacent region border which lies further out or an uneven number of large outer borders which are not yet connected and which also represent phase conflicts (B), the set of connecting paths generated is then reduced to that in which each phase conflict is contained precisely once (C), those regions (coverage regions) of the connecting paths which lie over the transparent regions are then marked (D), and finally, for the phase mask, the coverage regions are formed as region boundaries between two different regions of the phase mask which is to be manufactured and whose phase shifts have a phase difference of xcex94xcfx86=180xc2x0 (E).
Thus, in its first step, the present invention provides a formalism by means of which the possibility of directly connecting integrated semiconductor circuits into alternating phase masks, both dark-field masks and bright-field masks is checked. This is done by means of an explicit localization of the phase conflicts occurring in the respective layout by exclusively using the technological requirements which are made of the design. The set of phase conflicts which is determined using this formalism is complete and minimal, i.e. all the existing phase conflicts are always determined, and existing phase conflicts are not indicated repeatedly, for example.
These phase conflicts are then eliminated by means of the automated method described.
In the method step B., during the elimination of the phase conflicts (e) the connecting paths can be formed in a practical and rapid fashion both with dark-field and bright-field masks, in such a way that firstly pairs of edges, lying opposite one another, of in each case one large outer border and one directly adjacent region border, lying further out, are determined and then at least one connecting path is generated between the edges of each pair. However, because the connecting paths must later lead to phase jumps in the phase-shifting regions, only the following edges have to be taken into account in the group-based resolution of the associated adjacent position problem:
1) all the edges of the large outer contour of the respective layout group,
2) all the edges of the polygons which represent phase conflicts and from which the contact lines are to be removed.
However, the connecting paths do not necessarily run between edges of pairs of the large outer borders and of the layout group borders. Furthermore, a connecting path between two phase conflicts or between a phase conflict and the outer contour of a layout group does not necessarily need to be straight; it can instead run through lands with an even number of contact lines (that is to say no phase conflicts) and assume a very complicated shape. Its essential feature is that it connects a phase conflict either to the outer contour of a layout group or to an uneven number of the remaining phase conflicts which are not yet connected.
After the connecting paths have been generated, they are reduced in the method step D to a set of connecting paths in which each phase conflict occurs only once. For the inventive elimination of the phase conflicts it is thus sufficient to retain in each case just one connecting path between two phase conflicts or one phase conflict and the outer contour of its layout group after the reduction.
Preferably, specific technological requirements, for example the width of trim openings, continue to be predefined, the requirements being used as the basis on which a number of connecting paths which are firstly generated are also subsequently recognized as being invalid. As a result of this, there may still be phase conflicts without connecting paths because of the reduction and the invalidity after the method steps have been run through. In this case, in a second step, further adjacent elements, i.e. further external large outer borders or adjacent phase conflicts are taken into account. If there are still phase conflicts without connecting paths after this iteration, it is necessary to distort the layout in order to eliminate the respective critical structure, or a multiple exposure technique with phase masks as trim masks is necessary. Such a distortion or generation of the necessary trim masks can readily be automated on the basis of the uniquely defined localization of the phase conflicts which took place in the first part of the method according to the invention.
The following step in the method according to the invention is for those regions of the connecting paths which lie over the phase-shifting regions to be taken into account or marked in some way. These regions are optimum structures for automatically generating trim masks and for automatically carrying out the multiple-phase mask technique.
In the last step, in order to prepare the manufacture of the phase mask, the coverage regions are embodied as region boundaries between two different regions of the phase mask to be manufactured. For this purpose, for example, the coverage regions can be removed by the phase shifters and the regions of the phase mask to be manufactured can be represented in some suitable way, for example as two different colors of a phase mask which can be colored with two colors.
The connecting paths formed in this method, and thus also the coverage regions are preferably embodied as thin webs. During the projecting imaging of the phase mask, dark, unexposed lines are produced in the coverage regions as a result ofxe2x80x94in this case undesiredxe2x80x94interference between the adjacent regions which are phase shifted through 180xc2x0. In order to subsequently expose these dark lines, it is possible, for example after the method step F.), to manufacture a trim mask which has openings in the coverage regions so that these regions are subsequently exposed on the photoresist to be exposed.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for detecting and automatically eliminating phase conflicts on alternating phase masks, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.